Over 12,000 cases of the Acquired Immune Deficiency Syndrome (AIDS) have been diagnosed in the US resulting in over 5,000 deaths. Prospective studies have been undertaken in Central Africa to study the unique epidemiologic, virologic and immjunologic features of the disease which has occurred in several thousand Africans. Epidemiologic features. Over 400 cases of AIDS has been identified in Kinshasa over the past 8 Months (incidence rate: 380/106). Unlike the US, the male to female ratio is 1:1, and the disease is predominantely transmitted heterosexually. Household transmission studies demonstrate that 72% of spouses of AIDS patients were invected with HTLV-III virus compared to 4% of control households. Excluding spouses, 11% of household members of cases were seropositive compared to 3% of control households (p less than 0.05), suggesting other possible means of transmission. Studies of blood bank donors, health care workers, hospitalized pediatric patients and neonates suggest both vertical and needle trancmission. Virologic features. The ELISA for detection of antibody to HTLV-III has proven to be both sensitive (99%) and specific (99%) in studies of over 400 African AIDS patients and 5,000 healthy African controls. HTLV-III has been isolated from 27 of 35 (77%) patients with AIDS and from nearly 70% of seropositive non-AIDS patients. Genomic studies of Zairian isolates demonstrate marked heterogeneity compared to North American and Europena isolates. Further studies will examine differences in human and animal viral isolates in Kenya and Zair in order to study viral heterogeneity, and its relationship to clinical and immunologic features. Immunologic features. Seropositivity to HTLV-III correlated with marked depression of T4 + and T8 + lymphocytes and anergy. Patients with tuberculosis and malaria have increased activated T cell and a high prevalence of HTLV-III antibody (40% and 20%, respectively). Seropositivity in these diseases correlated with elevated T8 + cells followed by depressed T4 + cells. Further studies will examine whether endemic tropical diseases may increase susceptibility to HTLV-III infection and/or accelerate the development of opportunistic infections among seropositive individuals.